Ignition Timing: HEI-Est
All models are equipped with a High Energy Ignition system with Electronic Spark Timing (HEI-EST). The distributor contains a 7-terminal HEI-EST control module. The distributor is connected to the EST system by means of a 4-wire connector, leading to the external electronic control module (ECM).
When engine speed reaches 400 RPM or more (about 5-15 seconds after starting), the ECM transmits a constant 5 volt signal to the distributor HEI-EST module. This changes the position of the by-pass switch in the HEI-EST module.
When this occurs, the pick-up coil's signals no longer flow directly to the ignition coil. Instead, the RPM signals are routed to the ECM.
The Programmable Read Only Memory (PROM) portion of the ECM has a basic spark advance curve based on engine speed. Spark timing is calculated by the ECM whenever an ignition pulse is present. Spark advance is controlled only when the engine is running (not during cranking). Engine sensor values are used by the ECM to modify the PROM information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions.